Using an imaging plate exchanger and a synchrotron X-ray source, X-ray diffraction patterns were recorded from frog skeletal muscles shortening near the maximum velocity from a sarcomere length of 2.6 to 2.1 microns. The major findings were as follows: (1) the intensity of the second-order myosin meridional reflection, which decreased to 18% of the resting value during isometric contraction, increased to 34% during shortening, showing that the perturbation in the axial arrangement of myosin heads recovered. (2) The intensity of the third-order myosin meridional reflection increased by 32% on isometric contraction and decreased to 51% of the resting intensity during rapid shortening, showing disorder in the axial rearrangement of the myosin heads. (3) The axial Bragg spacing of the third-order meridional reflection, which increased by 1.22% during isometric contraction, was still 0.47% larger than the resting value during shortening, indicating that the increase is not entirely due to extension of the filament by tension. (4) The intensity ratio of the (1,0) and (1,1) equatorial reflections increased a small extent (from 0.57 to 0.62) during shortening, while the 5.9 nm and 5.1 nm actin layer-lines showed large intensity decreases (126 to 101% and 190 to 124% of the resting intensity, respectively), suggesting that considerable numbers of myosin heads are in the vicinity of the thin filament but not tightly bound to actin molecules during rapid shortening. These results are consistent with a model of muscle contraction in which myosin heads attach to actin in two different manners with weak and strong affinities and show that, during shortening, the number of tightly attached heads decreases by half, while the total number of attached heads decreases only slightly.